1. Field of the Invention
This invention relates to an intake air flow sensor for measuring the quantity of air sucked by an internal combustion engine.
2. Description of the Prior Art
Intake air flow sensors using a heat ray flow velocity meter are generally known well in the art and put into practical use as disclosed, for example, in Japanese Patent Laid-Open No. 162413/1984 laid-open on Sept. 13, 1984.
In the intake air flow sensor described above, an insulator for heat insulation is interposed between a sensor main body where a heat ray sensor is disposed in practice and a connection unit main body which is disposed downstream of the sensor main body.
In this case, the insulator defines part of the outlet of a bypass passage.
In the conventional intake flow sensor described above, however, an opening of a bypass passage to a main passage is formed between the sensor main body and the insulator in order to guide the air flowing through the bypass passage into the main passage and moreover, the sensor main body and the connection unit main body are integrated with each other by pressure-caulking or bolt fixing. Accordingly, the opening to the main passage is not compressed but is closed by the insulator so that the area of the opening of the bypass passage to the main passage changes.
The change in the area of the opening in turn changes the output characteristics of the intake air flow sensor disposed in the bypass passage when the intake air flow sensor is taken into consideration as a whole.